This is my first moderately successful convection (not fan-forced) turbo stove.
It is also the cheapest one. The top tube is a stainless steel toilet brush holder which cost $8 aus including the brush. The bottom is a milo tin, it was free but is going to rust out pretty quickly.
It works well some of the time but is even more temperamental than the fan forced ones.
With such a short stack there is very little updraught to suck air into the bottom and even less to pull air into the top holes. It rarely worked in a proper "turbo" mode with the top flame confined to the mixing area inside the top of the tube. Flames often went from the bottom to well above the top but still burned cleanly.
The chunks of fuel needed to be fairly large or they would restrict the airflow too much. Pellets didn't work well and nor did the charcoal phase. Increasing the height is sure to help but will require support.
I think pellet burning may be possible if a chimney is fitted. This would be useless for cooking but a good place heater.
This photo shows the gasification and after burn. You can see the chamber is full of smoke which is burning at the mixing zone. The smoke is consumed by the flame and the exhaust is clear as it leaves the stove at the top.
Adding a second section of tube improved the draw enough to burn pellets if I was careful.
The same can be achieve by drilling holes on a length of pipe.
A six metre length of 100mm stainless steel pipe was too expensive.
I bought a length of stainless flue instead for around $55. I couldn't buy smaller the 125 mm dia.
With care it would burn pellets and glow a satisfying orange. Burnt paper pellets are visible on the plate.
My tests showed that a fan-less stove can burn pellets but it is problematic. If the stove is overloaded and chokes it is very difficult to get it back to burning properly again.
Yet another one.
I guess I'm up to version 10.This is another fan forced stove. Convection doesn't really work for fuels like sawdust and pellets.
This one is very similar to mach-4 except it uses a stainless steel wine chiller instead of a flask.
The chiller doesn't have a removable end cap so it is simply pop riveted in place and not removable.
I used the same type of two watt 40mm fan as before and ran it from a 12V gel-cell battery.
The wide mouth is more convenient for adding fuel but as make it more sensitive to wind. There was a strong breeze blowing when I took these photos and sometimes the flame would partly blow out and release a little smoke - not much though.
This photo shows the chamber full of dense smoke which is completely burned up at the mouth of the stove.
As well as the easy to burn fuels (like wooden sticks) I successfully burned more difficult ones like chicken feed pellets, paper pellets and pine bark. The video chip below is similar to this still photo.
When burning a batch of fuel there are two distinct phases.
First you have the flaming pyrolysis phase where the chamber is full of dense smoke as in the above.
Then there is a rapid transition to the charcoal phase. For a brief period the gas in the chamber becomes transparent while still maintaining an orange flame. Here we can see though the flame and view the glowing coals.
The flame rapidly shrinks and changes to a pale pink. We are now burning carbon monoxide.
The flame is often colored differently because of impurities in the charcoal.
Paper pellets sometimes give off a beautiful blue just before the flame dies.
I often also see a faint whitish smoke at this point.
This flame tends to be weak and may go out. CO inhalation is dangerous so you should not do this in a confined space. This is true pretty much any stove burning carbon rich fuel.
This is the stove after the first test firing.
Note the holes near the rim.
These are approximately 4mm diameter. These are somewhat randomly spaced because my first attempt to make a jig to drill them failed.
I ultimately punched guide holes freehand uses an old augur bit and a hammer.
If I do this again I'll try making a punch with a guide attached - I think grinding an open ended spanner so one jaw is sharpened to a point might work. If you were doing a lot of this maybe you can make a tool similar to a puncture type can opener.
The main air holes are simply 10mm holes drilled through the bottom after the chiller is riveted in place.
These hole supply air to the main chamber and also to the top holes via the hollow walls.
If you wanted a greater proportion of the air going to the top you could add some holes which don't go all the way into the main chamber.
If you've followed the whole story from version one you can see there is a great variety of ways the build a clean burning turbo-stove or furnace. New sizes and shapes of flasks coolers and other potential stove parts are turning up in my local supermarket all the time.
Jan - 2008,
I'm working on a hiking stove. It is based in a 430ml hollow walled stainless steel travel mug. It is working quite well so far.
I can't keep building new stoves every time I see a new candidate but one day I plan to build a really big one.